1. Field of the Invention
This invention relates to a process for preparing an organopolysiloxane gum having a high degree of polymerization and a minimized content of low-molecular weight siloxanes.
2. Prior Art
Because of its weatherability, electrical properties, low compression set, heat resistance, low-temperature resistance, and the like, silicone rubber has been utilized in a wide variety of applications covering use in electronic equipment, automobiles, buildings, medical equipment and the food industry. Several exemplary applications are rubber contacts used as rubber contact keys in remote controllers, typewriters, word processors, computer terminals and musical instruments; building gaskets; developing, transfer, charging and paper feeding rolls in copying and printing machines; vibration dampers in audio equipment; and packings for holding compact disks in computers. The demand for silicone rubber is increasing and there is a desire to have silicone rubber having further improved properties.
Such silicone rubber is prepared from a raw material in the form of organopolysiloxane gum having a high degree of polymerization, which is prepared as disclosed in U.S. Pat. No. 2,546,036 by treating a liquid hydroxyl-terminated organosiloxane having a low-molecular weight with polydimethylsiloxane in the presence of sodium hydroxide catalyst. U.S. Pat. No. 2,634,252 also discloses catalysts which are useful in the preparation of siloxane gum from the standpoints of reaction rate, viscosity and quality, for example, potassium hydroxide, potassium silanolate, lithium silanolate, and tetraalkyl-ammonium hydroxide.
When these methods are employed, however, a substantial amount (5 to 10% by weight) of low-molecular weight cyclic siloxanes are left in the siloxane gum resulting from polymerization reaction. These low-molecular weight siloxanes adversely affect silicone rubber products. In the case of rubber contacts, for example, the amount of low-molecular weight siloxanes in a silicone rubber shape is reduced to further improve the durability and reliability of the rubber contact. More particularly, the low-molecular weight siloxane content is reduced by an enhanced heat treatment. In one exemplary method, the rubber contact shape is vulcanized and thereafter, heat treated in an oven at 150.degree. to 250.degree. C. over 2 to 24 hours. Such enhanced heat treatment is disadvantageous from the aspects of labor and cost.
For the same reason, roll materials used in copying machines are required to have reduced low-molecular weight siloxane content. In the case of building gaskets, it is known that the substance which causes staining of the surrounding building gasket is low-molecular weight siloxanes remaining in the siloxane gum of which the gasket is made. Under the circumstances, silicone rubber manufacturers attempted to reduce low-molecular weight siloxanes in starting siloxane gum rather than later removing low-molecular weight siloxanes from molded rubber.
One exemplary method for removing low-molecular weight siloxanes from a siloxane gum is by placing the gum under vacuum at elevated temperatures of 100.degree. to 300.degree. C. since the siloxane gum is highly viscous, however, great amounts of energy and time are required in order to achieve complete removal. Thus this method encounters a practical limit. It was also proposed to dissolve siloxane gum in a solvent such as toluene for extracting low-molecular weight siloxanes although this method added cumbersome steps and was disadvantageous in the industry.
There is a need for a polymerization process of preparing siloxane gum having an extremely low content of low-molecular weight siloxanes.